(1) Field of the Invention
The present invention relates to an electroconductive grease-filled bearing suitable for bearings at positions requiring an electrostatic discharge function and a high temperature durability.
(2) Related Art
Electrophotographic process machinery including electrostatic transfer copying machines such as digital PPC and color PPC, and printers of color LBP or color LED type system is now widely used. In the case of, for example, the former electrostatic transfer copying machines, it is a common practice to form electrostatic latent images on a photosensitive drum, then deposit toners thereon, transfer the visual images thus formed on the photosensitive drum onto printing paper by electrocharging a transfer electrode, and then forward the printing paper released from the photosensitive drum to a roll in a fixing section, thereby fixing the toners to the printing paper surface by heating and pressing. Static electricity is generated, for example, on the roll in the fixing section by passage of the printing paper through the printing course. As a means of discharging the static electricity to the outside, usually a discharging mechanism is provided by earthing the roll shaft end.
Such provision of the static electricity discharging system at the roll shaft end inevitably gives rise to an increase in number of additional parts. To reduce the number of such additional parts, so called “electroconductive bearing”, where the roll bearing per se is made electroconductive, has been so far used. It is well known that the electroconductive bearing includes such a type that electroconductive grease is filled in the interior of bearing or such a type that the bearing is provided with an electroconductive seal or an electroconductive member. Above all, the electroconductive grease-filled bearing requires neither additional special structural parts nor materials and thus is cost-wise advantageous, resulting much use in bearing positions requiring the electrostatic discharging.
The fixing section of the electrostatic transfer copying machine is a zone for fixing the toners electrostatically deposited on the printing paper by pressing at elevated temperatures, e.g. higher than 100° C., and thus support bearings of the rolls in the fixing section are usually operated at such elevated temperatures. Among the rolls in the fixing section, particularly the heating roll has an internally heating structure using a heater provided inside the hollow shaft, and the bearings that rotatably support the heating roll are more often exposed to elevated temperatures, e.g. 200° C. or higher.
That is, the bearings that support the rolls in the fixing section are required to have not only an electroconductive function to discharge the static electricity, but also a function of heat-resistant durability to operation at elevated temperatures. To meet the tendency of higher and higher temperatures due to higher speed operation of the machinery, much higher level is required even now for these two function.
Generally, the conventional electroconductive grease capable of satisfying such higher level required for the two functions contains carbon black as a thickening agent and electroconductive substances such as graphite, copper powder, etc. For example, a grease composition, which contains various kinds of carbon black as a thickening agent and an additive, etc. is disclosed in JP-A-57-3897. The grease composition has a good initial electoconductivity and can satisfy the electroconductive durability required for the ordinary grease, but the electroconductive grease must satisfy a prolonged electroconductive durability at elevated temperatures. However, carbon black with a structurally poor lubricability fails to satisfy such prolonged electroconductive durability requirement.
In a grease composition comprising a specific propylene polymer and an electroconductive component (including graphite), a method of decreasing a proportion of a thickening agent having an adverse effect on the electroconductance and adding a thickener (viscosity increasing agent) to assure the electroconductance is disclosed in JP-A-10-30096, where mineral oil or ester oil is used as a base oil, and thus the grease composition has a poor durability at elevated temperatures, as compared with the conventional fluorocarbon oil grease and also fails to satisfy the required functions when used in the above-mentioned zone.
Furthermore, the above-mentioned conventional grease compositions sometimes suffer from much leakage of base oil components from bearings due to separation of the base oil from the grease when used at elevated temperatures, e.g. 200° C. or higher, resulting in fouling at and around the bearings. Once the base oil component leaks from the bearings and deposits on the surfaces of the photosensitive drum or the rolls in the fixing section in the electrophotographic process machinery such as an electrostatic transfer copying machines and printers, the electrostatic latent images formed on the photosensitive drum will be disturbed, or the printing paper will be fouled in the fixing step, resulting in failure to satisfy the basic functions of electrophotographic process machinery. Furthermore, once separation of the base oil takes place, lubrication will be deteriorated in positions requiring the prolonged durability, resulting in failure to satisfy the required level of the functions.